The present invention relates to a new injection casting method using a mold which is kept under low pressure during filling, which method is more particularly intended for making:
cast pieces in aluminum, copper, iron, chromium or nickel alloys, PA1 cast pieces in plastic materials, PA1 cast pieces having parts in fibers treated by impregnation, such parts generally including thin portions. PA1 1) controlling the flow of metal, and PA1 2) filling a mold under vacuum with a metal under pressure. PA1 by controlling the speed of the metal flow from the pressure difference between furnace and mold, PA1 by allowing for the reducing height of the metal during the casting operation, PA1 by replacing the dynamic sensor by a static detection sensor which accurately defines the point of origin of the pressure variation inside the furnace right from the start of the cycle. PA1 Patent EP-2 0296 074, in which the two enclosures, one containing the metal and the other the mold, are placed under identical low pressures, an excess of pressure .DELTA.p being then created in the enclosure containing the metal to enable the latter to fill the mold, and being kept constant until the end of the solidification step. PA1 Patent FR-A-2 556 996, in which the mold and the enclosure containing the metal are, at the beginning, under the same low pressure, or at the atmospheric pressure. By creating a difference of pressure between the two enclosures, the metal is brought to the opening of the mold. From that point, the accuracy of which depends on the height of the metal inside the furnace, when there is no sensor, an excess pressure, with respect to the mold enclosure, is applied in the furnace, in order to fill the mold. Such excess pressure is obtained by introducing gases into the furnace through a pipe under a set pressure, and without any control. PA1 U.S. Pat. No. 5,042,561, in which the mold, which is under airless atmosphere and carries its injection tube, is placed on the furnace which is also under airless atmosphere. Both mold and furnace are, at the beginning, under atmospheric pressure. PA1 a) to ensure filling of the mold that is kept under low pressure or under vacuum, while observing predetermined speed values at every point of the mold, in order to create at said points conditions which will make it possible to obtain optimum metallurgic characteristics and to reproduce these characteristics from only one sensor located outside the mold and the metal stream. The sensor sends to the control apparatus signals indicating the position of the metal in the crucible at the start of the cycle, and during the cycle, in order to allow said apparatus to effect the necessary adjustments; PA1 b) to ensure a solidification under pressure in order to prevent any release of the occluded gases which could occur during a solidification under a vacuum, even if only a partial vacuum. Such passage of the metal from a state under vacuum to a state under pressure is achieved, according to the present invention, according to a predetermined variation rate to arrive at the required level of pressure. PA1 1) the mold enclosure and the crucible enclosure which, at the beginning, are under atmospheric pressure, are then brought to the same predetermined low pressure level, PA1 2) a metal level sensor measures the level and sends the information to the control apparatus, PA1 3) then a gas, adapted to the metal to be cast (such as air or nitrogen for aluminum alloys, and CO2+SF6, Argon+SF6 for magnesium alloys) is injected into the crucible enclosure, while the mold enclosure is kept to the vacuum level of the first step or to a level near thereof, which level changes as the gas releases in the mold due to the combustion of the resins from the sand. PA1 * on the one hand, the .DELTA.P between the pressure inside the mold and the pressure inside the crucible enclosure, PA1 * on the other hand, the difference between the quantity of metal contained in the crucible at the start of the cycle and the quantity present at the considered moment. Such reduction of the quantity of metal in the crucible may be determined, for example, by measuring the .DELTA.H of height of the free surface of the liquid metal between the considered moments or the difference of mass. PA1 of a low level (O to 1 bar, hence the designation of low pressure) for conventional castings, or PA1 of a high level (several tens of bars) for special cases such as for pieces comprising elements in composite materials with metallic matrix.
The conventional low pressure casting method has been known now since the beginning of the century: the metal is contained in a tight furnace, the mold is in contact with the metal via a tube. If the pressure is raised inside the furnace, the metal rises in the mold. After solidification, the pressure inside the furnace is reduced and the metal of the part which has not yet solidified is collected in the crucible.
The aim of the prior art has been essentially to bring two major improvements to the original method:
The control of the flow of metal is achieved by giving, in every point of the mold, to the advancing metal, a flowing speed which is high enough to prevent the advancing metal from solidifying before filling of the mold cavity is completed, and which is lower than the speed at which turbulences appear, which turbulences would cause the formation of oxides and gas pockets.
Therefore, at each point of the mold, the metal should have an optimum speed ranging between the two speeds discussed above; such speed is dependent on the geometry of the piece in that point, particularly on the thickness, on the nature of the mold (metallic, sand, ceramics), on the temperature of the metal and of the mold, on the nature of the metal, etc. . . .
The novelties which have resulted from the search for a solution represent three stages:
First stage: 1960/1975:
In French Patent No. 1 376 884, the pressure inside the furnace is controlled to reach values which are predetermined as a function of time. But since the height of the metal inside the crucible reduces as the casting operations follow one another, the predetermined value for a given time gives a different position inside the mold during a succession of castings. Moreover, no allowance is made for any adjustment caused by the temperature of the metal which affects its viscosity hence its flowing speed.
In French Patent No. 1 257 708, the method involves the use of a control device in order to obtain a constant pressure-increasing speed inside the furnace during filling of the mold cavity. Therefore there is no speed variation occurring during the casting as a function of the geometry of the different zones traversed by the metal. And moreover, just as before, there is no adjustment of the speed caused by the temperature.
French Patent No. 2 276 125 uses a low pressure casting installation associated with a calculator which ensures a pressure increase speed which is specific to each piece but constant up to the solidification threshold, the pressure of such solidification threshold being increased at each casting operation so as to take into account the weight of the metal used in the previous casting operation. Therefore, in this method, just as in the preceding ones, the flowing speed is not controlled as a function of the geometry of the part in its different zones. Also to be noted is the absence of adjustment due to temperature.
Second stage: 1972/1975:
French Patent No. 2 189 150 approaches the problem differently by allowing the flow over sensors, thus creating air flows of different values in order to vary the flowing speed of the metal, but such a control is affected by gas leaks which generally exist in industrial furnaces where tightness cannot be total. Therefore, predetermined speeds cannot be established by controlling air flows.
Third stage: 1979/1981:
To solve this problem of obtaining a predetermined flowing speed of the bead of metal in a given area, the Applicants have described in French Patent No. 7 917 317 and in European Patent No. 55947, the use of a reference sensor provided inside the stream of metal and detecting the passage of the metal. The pressure inside the furnace and the time taken by the metal to flow over the sensor are taken as being zero value in subsequent evolutions of the pressure inside the furnace. This makes it possible to overcome the effect of the height of the metal inside the crucible at the start of the cycle, a pressure zero and a time zero being always obtained at the same point, i.e. at the reference sensor.
This method, however, does not allow for three important factors, which are:
1) That the pressure can increase inside the mold during filling thereof due to the gases produced from the burning of the resins, porosity deficiencies in the sand, and the molds cast in low pressure conditions which are generally closed. But an open mold technique does exist.
The back pressure created by the gases reduces to the same value the pressure which causes the movement of the metal thereby reducing the speed thereof. This phenomenon occurs at random and the speed of the metal must therefore be controlled by the difference of pressure between the furnace and the mold, and not by the pressure of the furnace alone.
2) During the casting operation, the height of the metal in the crucible reduces and part of the pressure increase in the furnace is used for balancing said loss of height.
The predetermined speed to be obtained in a given point should therefore allow for such variation which may be very high for large pieces, even reaching up to one meter, i.e. being equivalent to the pressure used for filling the piece.
3) The reference sensor detecting the passage of the metal, described in Applicants' aforesaid patents is a dynamic detection sensor whose inertia, although low, causes the position of the point of the mold where the predetermined metal speed should be obtained, to shift.
It is the object of the present invention to provide a solution to said three problems relating to the control of the flow of metal:
Different solutions have been proposed to the second problem arising in the original low pressure casting method, i.e. that of filling a mold under vacuum with a metal under pressure, essentially by:
U.S. Pat. No. 2,997,756 and U.S. Pat. 1,703,739 describe the filling of a metallic mold to obtain an ingot, the mold being placed inside an enclosure in which the pressure is raised.
Japanese Patent Abstract No. 61 095 760, in which the metal and the mold are located in two enclosures under reduced pressure. Filling is achieved by increasing the pressure in the enclosure containing the metal.
An extra vacuum is created in the mold cavity in order to allow the metal to rise, which extra vacuum is kept up during solidification.
According to all the aforesaid methods, the metal fills up the mold at any speeds: a constant pressure is applied on the metal, or an extra vacuum is created in the mold cavity.
The object of the present invention is twofold. It is:
These objects are reached with the following steps:
A control apparatus receives at any moment the information supplied by the sensors, which latter provide said apparatus with:
Moreover, a quick response temperature sensor is provided at the opening of the mold and gives the temperature of the flowing metal, hence the .DELTA.T between the real temperature of the metal and the reference temperature set for the cast pieces as parameters. According to the control apparatus, the .DELTA.P corrected by the .DELTA.H and the .DELTA.T, takes on predetermined values to obtain the optimum fill speed in accordance with the invention.
4) When filling of the mold is completed, an extra pressure is applied on the metal according to a predetermined variation in order to ensure the "feeding" of the piece which is going to solidify, i.e. the rising of the liquid metal from the crucible and the casting system up to those areas which are going to contract during solidification.
5) Having determined said extra-pressure, the mold enclosure is rapidly brought to a pressure which is higher than or equal to the atmospheric pressure, and the crucible enclosure is brought to a predetermined pressure higher than that of the mold enclosure. This stage will be maintained up to the end of the solidification of the piece.
Such pressures in the two enclosures, during solidification of the piece, may be:
6) At the end of solidification, the two enclosures are depressurized and returned to the atmospheric pressure with predetermined speeds in order to prevent turbulences in the crucible when the metal flows back.
The crucible enclosure may also be kept under a residual pressure in order to keep the metal in the upper part of the tube.
During the variations of pressure after filling of the mold, it is the .DELTA.P between the crucible enclosure and the mold enclosure which is used as a basis for controlling the pressure inside the furnace by the control apparatus, since the pressure sensor has become inoperative after the filling operation.
The following special cases should be noted:
A zero vacuum level corresponds to the conventional low pressure casting with the mold and the metal being at the atmospheric pressure at the start of the injection. The method then specially controls the flowing speed of the metal and of the extra pressure with adjustment due to the height of the metal inside the crucible and to the temperature at the entrance into the mold.
A level of vacuum above 500 millibars is used for magnesium alloys, which level can vary as a function of the nature of the alloy.
At the end of the mold-filling operation, it is possible not to have to apply extra pressures, to have feeder heads open at the top, and to achieve solidification under gravity of the upper part of the piece and solidification under pressure of the lower part.
The method, in this case, provides the control of the speed during filling, and the keeping of the metal at the higher level during solidification.